Sorry, I should have included this in my last post. My kittens are 4 weeks old and still not showing any signs of peeing or pooping. Not even little surprises left for us. I'm assuming the mother is still eating them. What age are they supposed to use litter box and what is a good way to introduce them so they get used to it?

Don't worry. That's what moms are for. She will show them how when they are ready. Be sure to have a largish low-sided box handy for them so they can get into it easily.